Stepping into the political minefield of immigration reform, Homeland Security Secretary Janet Napolitano soon will direct federal agents to emphasize targeting US employers for arrest and prosecution in connection with the illegal laborers who sneak into the country to work for them, department officials said Monday.

WASHINGTON — Stepping into the political minefield of immigration reform, Homeland Security Secretary Janet Napolitano soon will direct federal agents to emphasize targeting U.S. employers for arrest and prosecution in connection with the illegal laborers who sneak into the country to work for them, department officials said Monday.

The shift in emphasis will be outlined in revamped field guidelines issued to Immigration and Customs Enforcement agents as early as this week, several officials familiar with the change said. The policy is in keeping with comments that President Barack Obama made during his campaign for office, when he said past enforcement efforts failed because they focused on illegal immigrants rather than the companies that hired them.

"There is a supply side and a demand side," one Homeland Security official said. "Like other law-enforcement philosophies, there is a belief that by focusing more on the demand side, you cut off the supply."

Another DHS official said the changes were the result of a broad review of immigration and border-security programs and policies that Napolitano launched in her first days in office. "She is focused on using our limited resources to the greatest effect, targeting criminal aliens and employers that flout our laws and deliberately cultivate an illegal work force," that official said.

Homeland Security officials emphasized that the department would not stop conducting sweeps of businesses while more structural changes to U.S. immigration law and policy were being contemplated.

Agents, however, will be held to a higher standard of probable cause for conducting raids, the officials said, out of concern that at least one recent raid in Washington state and another pending sweep in Chicago were based on speculative information that illegal workers were there.

The officials spoke on the condition of anonymity because they were not authorized to discuss the policy changes.

The new guidelines would mark a fundamental shift away from what was happening toward the end of the Bush administration, said Doris Meissner, who served as commissioner of ICE's predecessor, the Immigration and Naturalization Service, in the Clinton administration.

The law governing employer enforcement requires proof that a business knowingly hired illegal workers. So without an effective way for employers to verify workers' status, Meissner said, "it is very easy for that 'knowingly' to be a big loophole."

Meissner, now a senior fellow at the Migration Policy Institute think tank in Washington, said the Bush administration also vowed to go after employers but rarely did so. In later years, it drew fierce criticism by conducting large-scale raids targeting workers.

The Clinton administration, in contrast, used a combination of laws to go after employers for smuggling, violating labor laws or engaging in criminal conspiracy, she said. "At the end of the day, when you make cases like that, you have more impact."

Advocates on both sides of the issue have been awaiting major changes in immigration policy since Obama's election — particularly since he selected Napolitano, a former border-state governor and prosecutor, to head Homeland Security.

Conservatives have warned that any easing of enforcement efforts against illegal workers would result in more coming into the country and competing for jobs. Immigrant-rights groups have complained that the lack of reform measures so far under Obama suggested the White House was backing down from campaign pledges to curb work-site enforcement efforts. Those concerns ratcheted up dramatically when ICE agents swept into a manufacturing plant in Bellingham, Wash., in February and arrested dozens of people on suspicion that they were in the country illegally.

Napolitano suggested to Congress that she was unhappy with the raid and that she would "get to the bottom of this." But, she added: "In my view, we have to do workplace enforcement. It needs to be focused on employers who intentionally and knowingly exploit the illegal labor market."

Homeland Security officials confirmed that a planned raid in the Chicago area was delayed in recent weeks because senior administrators now expected "a higher level of scrutiny to be applied," one official said. "Politics has nothing to do with it. It is all about the quality of the investigative work and the effectiveness of targeting the employers."

Michael W. Cutler, a retired senior special INS agent, said the Obama administration needed to go after both workers and employers to send a message that it would not condone illegal immigration.

"Who is more responsible for prostitution, the hookers or the johns? It is a shared responsibility," said Cutler, a fellow at the Center for Immigration Studies, an anti-illegal-immigration group. He said it would be "dumb" to "go after employers and not the illegal aliens. That means they are going to make very few arrests. And the message that sends is that if you can make it across the border, you're home free, no one is going to be looking for you."